Methanotrophic bacteria utilize methane as their sole source of carbon and energy. The first step in their metabolic pathway is the oxidation of methane to methanol by methane monooxygenase (MMO) enzyme systems. As the only biological methane sink in an era of increased methane emissions, methanotrophs are a potential means to mitigate the deleterious effects of global warming on human health. In addition, methanotrophs can oxidize substrates besides methane, including halogenated hydrocarbons, and have been targeted for bioremediation applications. The predominant MMO in nature is particulate methane monooxygenase (pMMO), an oligomeric, integral membrane metalloenzyme. The long term goal of this project is to understand how pMMO and its homologs activate O2 for oxidation of methane and other substrates. Despite the recent identification of the pMMO active site, major questions surrounding pMMO structure and function remain to be addressed. The proposed research will focus on elucidating the atomic structure of the copper active site, the nature of the reactive species that oxidizes methane, the details of substrate, product, and reductant binding sites, and the role of the transmembrane domains. In addition, characterization of unique pMMO homologs will be pursued. The experimental approach involves spectroscopic, mechanistic, biochemical, and crystallographic characterization of native pMMO, recombinant pMMO, recombinant pMMO subunits and domains, and recombinant domains from related enzymes.